CHEMICAL-MECHANICAL POLISHING AND THIN FILMS
The changes in the film surface hardness in the presence of different oxidizing chemicals are being measured using nanoindentation. These changes have a strong influence on the quality of the planarized film surface and the choice of an abrasive. Measurements, both in air and in the chemical environments of interest to CMP, are being performed. Electrochemical measurements, both in situ and ex situ, are providing valuable insights into material removal mechanisms. Ionic strength and pH have been shown to play an important role in determining removal rates of Cu and Ta films in different polishing environments. Polishing of Cu and Ta with silica abrasives of different surface areas suggests that the surface hydroxyl groups play an important role in material removal. Cu and Ta removal rates increase with increasing surface area of the silica abrasives. Lowering the surface hydroxyl concentration by partially replacing them with methyl groups lowered the removal rates, with the effect being more pronounced for Ta than for Cu. In one set of experiments, it was found that about 75 % of the removed Ta is attached to the silica particles while only about 44% of removed Cu is. In contrast, when alumina abrasives are used, about 88% of the removed Cu is present in solution. These experiments are being done in collaboration with Professor Li's group.
The effects of abrasive shape, size and morphology in CMP are being investigated in collaboration with Professor Matijevic¢. With support from SRC, well-defined dispersions using monodispersed spherical silica particles, silica particles coated with aluminum (hydrous) oxide, ellipsoidal particles of different anisometries coated with silica, and silica particles coated with ceria are being prepared and will be evaluated as abrasives in CMP.
A Westech 372, donated by Intel, has been installed and is providing a sorely needed extension to CAMP's planarization capabilities to be further extended soon by a 676 Orbital tool provided by Speedfam/IPEC. Also, IBM has donated two Westech 4100 polisher/brush cleaner systems.
Current research sponsors include New York State Energy Research and Development Authority (NYSERDA), SRC/Intel, Grace Davison, St. Gobain, PPG, Nyacol, Rodel, and Kodak, Inc. Professor Babu's research group now consists of five Ph.D. and three M.S. students.
Professor Cetin Cetinkaya's ongoing experimental work includes laser-based particle removal, ultrasonic methods for particle characterization and air-coupled ultrasonic evaluation. He is also interested in linear and nonlinear elastic and thermoelastic wave propagation and its applications in non-contact submicron and nanoparticle removal, mechanical particle-surface bond characterization and ultrasonic testing and evaluation.
With the International Technology Roadmap for Semiconductors decreasing particle removal requirements from 125 nm (0.3-0.75 per cm2) in 1997 to 25 nm (0.01 - 0.15 per cm2) in 2011, an era of the most challenging cleaning applications in semiconductor manufacturing is upon us. Professor Ahmed Busnaina and his group are addressing this problem. In order to meet these removal requirements, there is a need to understand the particle removal mechanisms and their limitations. See Figure 4.
Professor Busnaina and his group have been investigating the effect of decreasing particle size down to the nanoscale and its effect on the practical use of present techniques in the future. The study shows that the removal of nanosize particles (10-100 nm) can be best accomplished using acoustic streaming at frequencies larger than 1 MHz. The theory shows that the removal of silica particles in the nano-size (10-100 nm) range is achievable. It is also shown that megasonic wave induced acoustic streaming is essential to the removal of submicron particles. As the frequency increases, the acoustic boundary layer thickness decreases and the streaming velocity increases thereby increasing the drag force and consequently the removal moment. The results show that the removal moment has to overcome the adhesion moment. The force required to remove a particle by rolling is less than that required by sliding or lifting. The experimental results also show that complete removal down to 100 nm particles is achievable using a single wafer non-contact cleaning process with DI water only.